1.1A Technical Field
This disclosure relates to fail-safe systems, and in particular to a microprocessor-based system and method for providing fail-safe-control and monitoring of devices.
1.1B Description of the Related Art
Many critical applications of monitoring and control systems require that their operation be fail-safe in order to avoid human injury and property damage. To achieve this, the behavior of these systems are, under all possible conditions, to be predictable to an extremely high level of certainty. The fail-safety of such systems that employ electronic components has been the subject of considerable concern and study. A major source of this concern is the fact that electronic components typically exhibit failure modes that are random and unpredictable. A transistor, for example, either may fail electrically by being open-circuited or short-circuited. A device controlled by such a transistor may therefore fail in a state that is at best, undesired, or at worst, unsafe. Similarly, a failed component in a monitoring circuit acting as an input to a system may cause the system to operate with the input device in the state opposite to its actual state, which may result in unsafe operation.
A second potential source of unsafe system operation relates to the physical interconnection of input and output devices to a microprocessor-based controller that monitors and/or controls them. If the wires that connect to the devices are installed improperly, or if they come in improper contact with each other, the inputs being monitored may be misread or the outputs being controlled may attain an unsafe state.
In the design of control systems, techniques aimed at providing fail-safety by reducing the probability of wrong-side failures tend to increase complexity. This often results in reduced reliability because of the likelihood that one of the added components may fail. Therefore, a trade-off relationship often exists between fail-safety and reliability in that concepts intended to enhance one tend to compromise the other.
The illustrative embodiments of the hardware and programming specified herein address both of the above-mentioned problems of the prior art to provide completely fail-safe systems, and also address the problem of maintaining a high level of reliability.
Accordingly, I have invented a fail-safe system and method which use readily available, generic microprocessor-based equipment that, individually, are not inherently designed to be fail-safe.
A fail-safe system is disclosed for monitoring or controlling a device having at least first and second contacts, with the fail-safe system having a processor which operates using a program for generating an input signal, for applying the input signal to the first contact, for detecting a response signal at the second contact, and for comparing the response signal with the input signal to monitor the operational condition of the device, or to control the device.
In particular, the fail-safety of the overall system is secured by the interaction of specific hardware and programs such that each supports the other in a synergistic manner.